There is known a vehicle seat having a structure in which a seat back includes a top plate main part for supporting a body of a seated person from the rear side and a top plate side part protruding in a shape expanding forward from both left and right sides of the top plate main part to support the body of the seated person from both outer sides JP-U-H05-076860). The seat back has a configuration in which the left and right top plate side parts form a continuous shell-like shape independent from the top plate main part and can be moved rearward with respect to the top plate main part when getting on and off the vehicle.
In the above technology, when the top plate side parts are retracted at the time of being used for seating, the support for the body of the seated person from the lateral side is lost. Therefore, it is difficult to appropriately protect the body of the seated person at the time of occurrence of a side collision of the vehicle. The disclosure has been made as a solution to the above problem and an object to be achieved by the disclosure is to make it possible to appropriately protect the body of the seated person from the lateral side at the time of occurrence of a side collision of a vehicle, even when the top plate side parts can be retracted at the time of being used for seating.